AC/DC power conversion is used in many industrial, commercial, and personal electronic applications. AC/DC conversion inherently involves some inefficiency in terms of power lost between the AC input and the DC output. While some of this inefficiency is inescapable, some inefficiency may also be due to a phase angle difference between voltage and current, due to inductance and/or capacitance that reacts against the alternating current, which may be reduced or eliminated with a power factor corrector (PFC).
An important class of AC/DC converters are known as switched-mode power supply (SMPS) converters. An SMPS converter may use a boost converter PFC at the front end of an AC/DC power supply to shape the AC input current to correct the power factor (PF) and achieve a PF ideally as close as possible to 1 or unity, i.e., to reduce or eliminate the phase angle difference between the voltage and current. Another undesirable effect of power conversion involves total harmonic distortion (THD), and design factors that increase PF often also involve reducing THD. Design considerations that both increase PF and reduce THD may be collectively considered as improving PF/THD performance.
Power converters include a number of design constraints that involve trade-offs with PF/THD performance, such as switching voltage and electromagnetic interference (EMI) noise correction. Power converters are often designed with features that reduce switching voltage and reduce EMI noise, at the expense of reducing PF/THD performance. Optimizing among the trade-offs involved in addressing these various constraints is further complicated by dealing with variable loads that spend much of their operating time at a light load, drawing a fraction of their peak current. In many popular applications of AC/DC power supplies with moderate power ratings (e.g., under around 300 watts), such as TVs, laptop computers, and desktop computers, the electrical load may vary considerably, and the application may spend large amounts of time operating at a fraction of its peak electrical load. Typically, the lower the operating load, the more exacerbated the drawbacks in PF/THD performance. A PFC may be designed to mitigate the drawbacks in PF/THD performance by using a controller that enables operating modes with different switch timing techniques in the AC to DC switching. These may include critical conduction mode (CrCM) and discontinuous conduction mode (DCM). A PFC controller may also react to different loads and apply CrCM or DCM based on changes in the load, and in this case is known as multi-mode.